Causes of autism spectrum disorder

The exact cause of autism spectrum disorder (ASD) is currently unknown.

It's a complex condition and may occur as a result of genetic predisposition (a natural tendency), environmental or unknown factors.

Genes

Most researchers believe that certain genes a child inherits from their parents could make them more vulnerable to developing ASD.

Cases of ASD have been known to run in families. For example, younger siblings of children with ASD can also develop the condition, and it's common for identical twins to both develop ASD.

No specific genes linked to ASD have been identified, but it may be a presenting feature of some rare genetic syndromes, including Fragile X syndrome, Williams syndrome and Angelman syndrome.

Environmental triggers

Some researchers believe that a person born with a genetic vulnerability to ASD only develops the condition if they're exposed to a specific environmental trigger.

Possible triggers include being born prematurely (before 35 weeks of pregnancy), or being exposed in the womb to alcohol or to certain medication, such as sodium valproate (sometimes used to treat epilepsy during pregnancy).

No conclusive evidence has been found linking pollution or maternal infections in pregnancy with an increased risk of ASD.

Other health conditions

Below are some other conditions known to be associated with ASD:

muscular dystrophy – a group of inherited genetic conditions that gradually cause the muscles to weaken

Down's syndrome – a genetic condition that typically causes a learning disability and a range of physical features